My invention relates generally to the art of ultrasonic machining, and more specifically to ultrasonic lapping apparatus for refining the surface finish and geometrical accuracy of holes in drawing dies and, in particular, in such dies made from diamond or like hard material for drawing fine wires.
For drawing small sizes of wire, dies have been used extensively which are made from diamond, agate or the like or from artificial materials of greater hardness which have been recently developed. When they are properly made, the use of these dies is economical as they retain their accuracy for a long time. The hole in the wiredrawing die, through which the wire is drawn, usually comprises three major zones, the first being a tapered or cone-shaped approach at the entrance end where the actual reduction takes place in size of the wire. The second is a short cylindrical zone known as the bearing or land, where the roundness and size of the wire is determined. Then comes the third zone in the form of a reverse taper at the exit end, sometimes referred to as a back relief, which strengthens the exit end of the die and prevents the end of the bearing from spalling.
It has been known to employ the ultrasonic machining method to lap or finish the internal surfaces of the wiredrawing dies in question, as the method is applicable expeditiously to holes of extremely small diameters. The ultrasonic die lapping apparatus as heretofore suggested and used, however, is subject to the disadvantage that it depends much on manual assistance from the skilled operator to guide the lapping tool along the internal die surfaces of intricate shape. The operator of the conventional apparatus usually needs an experience of at least five years or so, and even then he may sometimes fail to operate the apparatus with truly satisfactory results.
The finishing operation of the wiredrawing dies has been highly time-consuming. For lapping a die with a bearing diameter of 0.9 millimeter to a desired degree of finish by use of the conventional apparatus, about one hour is required if the die is made from diamond, and well over ten hours if it is made from artificial material of greater hardness. The dies of artificial material, moreover, are difficult to lap to as smooth a finish as diamond dies, perhaps by reasons of the extreme hardness and crystal structure of the artificial material. The conventional ultrasonic lapping apparatus has thus been in need of substantial improvement to establish its true practical utility in conjunction with the drawing dies of the class under consideration.